Vertical-cavity surface-emitting lasers (VCSELs) are semiconductor lasers in which light emission occurs perpendicularly to the surface of the semiconductor chip. Vertical-cavity surface-emitting laser diodes have a plurality of advantages over conventional edge-emitting laser diodes, such as low electrical power consumption, the possibility of directly inspecting the laser diode on the wafer, simple possibilities for coupling to a fiber optic, production of longitudinal single-mode spectra and the possibility of connecting the surface-emitting laser diodes to a two-dimensional matrix.
In the field of fiberoptic communication technology, there is a need, due to wavelength-dependent dispersion and absorption, for VCSELs in a wavelength range from approximately 1.3 to 2 μm, and in particular for wavelengths of around 1.31 μm or 1.55 μm. Long-wavelength laser diodes, especially for the wavelength range above 1.3 μm, have been produced from InP-based compound semiconductors. GaAs-based VCSELs are suitable for the short-wavelength range of less than 1.3 μm.
The lateral beam profile of laser diodes may be influenced to a significant extent by the definition of the corresponding waveguide. In the case of GaAs-based VCSELs having emission wavelengths below approximately 1.3 μm, the waveguiding is produced by selectively oxidised Al(Ga)As layers (cf. “Electrically Pumped 10 Gbit/s MOVPE Grown Monolithic 1.3 μm VCSEL with GaInNAs Active Region”, Electronics Letters, Vol. 38, No. 7 (28 Mar. 2002), pages 322 to 324).
By far the best results, in terms of power, operating temperature, single-mode power and modulation bandwidth, for long-wavelength VCSELs in the wavelength range above 1.3 μm, are obtained with InP-based BTJ (Buried Tunnel Junction) VCSELs.
The production and structure of the buried tunnel contact will be described, by way of example, with reference to FIG. 1. Molecular Beam Epitaxy (MBE) is used to produce a highly doped p+/n+ layer pair 101, 102 having a low band gap. The tunnel contact 103 is formed between these two layers. A circular or elliptical region, which is formed substantially by the n+-doped layer 102, the tunnel contact 103 and a portion of or the entire p+-doped layer 101, is shaped by Reactive Ion Etching (RIE). In a second epitaxy cycle, this region is overgrown with n-doped InP (layer 104), so the tunnel contact is “buried”. The contact region between the overgrown layer 104 and the p+-doped layer 101 acts as a barrier layer on application of a voltage. The current flows through the tunnel contact at resistances of typically 3×10−6Ω cm2. The flow of current can thus be restricted to the actual region of the active zone 108. The amount of heat generated is also low, as the current flows from a high-resistance p-doped layer to a low-resistance n-doped layer.
The overgrowing of the tunnel contact leads to slight variations in thickness of the layers located thereabove, as illustrated in FIG. 2, and this has a detrimental effect on lateral waveguiding. In particular, the formation of higher lateral modes is facilitated, especially in relatively large apertures. Only small apertures, and a correspondingly low laser power, may therefore be used for the single-mode operation of conventional VCSELs; this mode is required, for example, in fiberoptic communication technology.
The complete structure of an InP-based VCSEL will now be described, by way of example, with reference to FIG. 2. In this structure, the buried tunnel junction (BTJ) is inverted relative to FIG. 1, so the active zone 106 is located above the tunnel contact, which has a diameter DBTJ and is disposed between the p+-doped layer 101 and the n+-doped 102. The laser radiation emerges in the direction indicated by arrow 116. The active zone 106 is surrounded by a p-doped layer 105 (for example, InAlAs) and by an n-doped layer 108 (for example, InAlAs). The leading-side mirror 109 above the active zone 106 consists of an epitaxial DBR (Distributed Bragg Reflector) comprising approximately 35 InGa(Al)As/InAlAs layer pairs, thus producing a reflectivity of approximately 99.4%. The trailing-side mirror 112 consists of a stack of dielectric layers as the DBR and is completed by a gold layer, thus producing a reflectivity of almost 99.75%. An insulating layer 113 is used for lateral insulation. An annularly structured further p-side contact layer 111 is provided between the layer 104 and the contact layer 114. FIG. 2 illustrates the manner in which the structure of the overgrown tunnel contact is propagated (in this case, downwardly) into the further layers.
The combination of the dielectric mirror 112 and the integrated contact layer 114 and the heat sink 115 results in a markedly increased thermal conductivity compared to epitaxial multilayer structures. Current is injected via the contact layer 114 or via the integrated heat sink 115 and the n-side contact points 110. For further details regarding the production and the characteristics of such VCSEL types, reference is expressly made to the following citations.
A VCSEL having the structure illustrated in FIG. 2 forms the subject-matter of the publication “Low-Threshold Index-Guided 1.5 μm Long-Wavelength Vertical-Cavity Surface-Emitting Laser with High Efficiency”, Applied Physics Letters, Vol. 76, No. 16 (17 Apr. 2000), pages 2179 to 2181. A VCSEL of the same type having an output power of up to 7 mW (20° C., CW) is presented in “Vertical-Cavity Surface-Emitting Laser Diodes at 1.55 μm with Large Output Power and High Operation Temperature”, Electronics Letters, Vol. 37, No. 21 (11 Oct. 2001), pages 1295 to 1296. The publication “90° C. Continuous-Wave Operation of 1.83-μm Vertical-Cavity Surface-Emitting Lasers”, IEEE Photonics Technology Letters, Vol. 12, No. 11 (November 2000), pages 1435 to 1437, relates to a 1.83-μm InGaAlAs—InP VCSEL. “High-Speed Data Transmission with 1.55 μm Vertical-Cavity Surface-Emitting Lasers”, Post-Deadline Papers, 28th European Conference on Optical Communication (8 to 12 Sep. 2002) discusses the use of a BTJ-VCSEL for error-free data transmission at modulation frequencies of up to 10 Gbit/s. Finally, a VCSEL having an emission wavelength of 2.01 μm (CW) is known from “Electrically Pumped Room Temperature CW-VCSELs with Emission Wavelengths of 2 μm”, Electronics Letters, Vol. 39, No. 1 (9 Jan. 2003), pages 57 to 58.
In contrast to the GaAs-based VCSELs having emission wavelengths below 1.3 μm, the lateral oxidation method may not be used in the BTJ-VCSELs under discussion, since the materials that are used have excessively low aluminium contents, and other conceivable materials, such as AlAsSb, have, to date, not yielded oxide layers of sufficient quality. In the above-discussed BTJ-VCSELs, the lateral waveguiding resulting from the production process accordingly takes place by lateral variation of the length of the resonator. Alternatively, selectively etched-off layers (cf. “1.55-μm InP-lattice-matched VCSELs with AlGaAsSb—AlAsSb DBRs”, IEEE Journal on Selected Topics in Quantum Electronics, Vol. 7, No. 2 (March/April 2001), pages 224 to 230), proton implantation (cf. “Metamorphic DBR and Tunnel-Junction Injection: a CW RT Monolithic Long-Wavelength VCSEL”, IEEE Journal on Selected Topics in Quantum Electronics, Vol. 5, No. 3 (May/June 1999), pages 520 to 529) or selectively oxidised metamorphic AlAs layers (cf. “1.5-1.6 μm VCSEL for Metro WDM Applications”, 2001 International Conference on Indium Phosphide and Related Materials, Conference Proceedings, 13th IPRM (14 to 18 May 2001), Nara, Japan) have, for example, been used in other long-wavelength VCSEL designs.
FIG. 3 illustrates schematically and not to scale the conditions in a known structure of a generic surface-emitting semiconductor laser. The diagram shows the borderline region between the current-carrying layer 7 and an n-doped contact layer 8 having a thickness d3, through which the current is generally supplied and which has preferably grown onto the layer 7 from highly n-doped InGaAs. The raised portion 15 is formed by the overgrowing of the tunnel contact and has a thickness d2 (=raised portion depth). The contact layer 8 is conventionally applied in an epitaxy step and removed by selective etching in the region of the raised portion 15. The structured contact layer 8 typically has a thickness d3 from 50 to 100 nm, to ensure low contact resistances, and, at its inner edge, is at a distance of a plurality of micrometers (typically 4 to 5 μm) from the tunnel contact raised portion 15. In the illustrated structure, the length of the resonator is greater by d2 in the center than in the regions outside the raised portion 15. The effective index of refraction is higher (typically by 1%) in the center than in the outer region, thus resulting in strong index guiding. This favours the formation of higher modes, especially in large apertures.